Tandem Mass Spectrometry: Reading a Peptide Sequence From Fragment Ions

A mass spectrum tells you what a peptide weighs. Tandem mass spectrometry — MS/MS — breaks that peptide apart inside the instrument and weighs the pieces, and because the pieces share a common backbone the differences between them spell out the sequence one residue at a time.

What the second stage adds

A single-stage measurement reports a mass. Two different sequences built from the same set of residues have identical masses, and a mass alone cannot separate them. The second stage exists to break that tie.

The instrument selects one ion out of the mixture entering it, fragments that ion, and records the masses of the fragments. Because only the selected ion was fragmented, everything in the second spectrum belongs to one species — which is also why MS/MS works on a mixture that a single-stage spectrum would report as an uninterpretable overlap.

Selecting the precursor

Selection is done by mass, over a narrow window — often one to three mass units wide. The window matters. Too wide and a co-eluting species of similar mass is fragmented alongside the target, producing a spectrum that mixes two sequences. Too narrow and the isotope envelope is clipped, cutting the signal.

The choice of charge state matters too. Multiply charged ions fragment more informatively than singly charged ones, which is part of why electrospray is the usual front end here — the difference between the two ionisation approaches is covered in MALDI versus electrospray for peptides, and the arithmetic of converting a multiply charged series back to a neutral mass in deconvoluting multiply charged spectra.

How the backbone breaks

Collision with an inert gas puts energy into the ion, and the peptide backbone breaks preferentially at the amide bond. Each break produces two fragments, and which one keeps the charge determines what is observed:

• b ions retain the N-terminal portion.
• y ions retain the C-terminal portion.

Breaking at every amide bond along the chain produces a full series of each. A b ion series is a set of masses that grows by one residue at a time from the N-terminus; the y series grows from the C-terminus. The two series are complementary — a b ion and its partner y ion sum to the precursor mass plus the mass of water and a proton.

Reading the ladder

The sequence is not read off any single peak. It is read off the gaps between consecutive peaks in a series. A gap of 71.037 is alanine. A gap of 128.059 is glutamine, 128.095 is lysine, 113.084 is leucine or isoleucine.

Those numbers are residue masses, not free amino acid masses — the residue mass is the amino acid minus water, because forming the peptide bond released it. The distinction between monoisotopic and average values applies here as strictly as anywhere, and at the resolution needed to separate glutamine from lysine only monoisotopic masses are usable; see monoisotopic versus average mass.

Other fragment types, and why they exist

Collisional fragmentation also produces a and c ions from the N-terminal side and x and z ions from the C-terminal side, in smaller quantities, plus internal fragments from chains that broke twice and immonium ions that report which residues are present without saying where.

Alternative dissociation methods change the pattern deliberately. Electron-based fragmentation cleaves a different backbone bond, yielding c and z ions instead of b and y, and it does so without disturbing labile modifications — a phosphate or a glycan that a collisional experiment strips off before the backbone breaks survives to be localised. It also leaves disulfide bonds intact or cleaves them selectively, which is why it appears in connectivity work of the kind described in disulfide formation and scrambling.

What the spectrum cannot distinguish

Three limits are structural rather than instrumental:

• Leucine and isoleucine are isomers. Standard fragmentation cannot separate them, and a reported sequence resolves them from the expected sequence rather than from the data.
• D and L residues are identical in mass at every stage. Chirality requires a separate method entirely, as set out in racemisation and chiral purity.
• Glutamine and lysine differ by 0.036, and glutamate and an oxidised residue can present similar ambiguities — all resolvable, but only at sufficient mass accuracy, which is the subject of mass accuracy in parts per million.

Coverage, again

A complete ion series is uncommon. Prolines suppress cleavage on one side and enhance it on the other, producing gaps and unusually intense peaks. Some bonds simply do not break under the energy applied. A spectrum that yields eight of twelve possible y ions has confirmed eight junctions and left four inferred from mass balance.

The honest way to report this is the same as for a digest map, and for the same reason — see peptide mapping by protease digestion: state which segments the fragment ions covered, and which were assigned by difference rather than observed directly.

Where it sits relative to the rest of the certificate

Sequencing by MS/MS is a confirmation of identity, not a measure of purity. It says the selected species has the expected residue order; it says nothing about how much of the vial that species represents, which remains the job of the chromatogram and the arithmetic described in what area percentage measures.

It is uncommon on routine peptide certificates for the same proportionality reason as digest mapping — for a short chain the intact mass plus a purity trace is usually the appropriate level of evidence. It becomes the deciding method when two candidate sequences share a mass, when a modification has to be placed at a residue, or when the material is long enough that intact mass no longer constrains the answer.